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Sunday, January 29, 2017

Explorer of Enceladus and Titan

The
search for another world with life in our solar system has arguably become the
most powerful theme in planetary exploration.While microbial life – the most likely form of life elsewhere in the
solar system – itself is likely to be hard to prove with a unified series of
past, current, and planned missions to Mars.NASA is developing an orbiter to investigate the habitability of Europa
and studying a follow on lander to directly search for life.And for the third time in less than a decade,
scientists have proposed a multiple-flyby mission to explore the habitability
of Saturn’s ocean moons Titan and Enceladus.

The
latest proposal, led by European scientists, is called Explorer of Enceladus and
Titan (E2T).It builds on the
experience gained from two less ambitious previous proposals, led by American
scientists, the Journey to Enceladus (JET) in 2010 and the Enceladus Life
Finder (ELF) in 2014.

I
met the principle investigator for the E2T proposal, Giuseppe Mitri
with the University of Nantes, at a conference in December.We discussed his team’s proposal and he has
subsequently provided me we a comprehensive look at the goals and the mission
implementation.In an email to me, he
explained the motivation for the mission: “Enceladus and Titan are two
unique worlds in the Solar System not only in terms of their geology and
evolution but also for their habitability. Flyby missions such as E2T provide
an unprecedented opportunity to explore in detail, surpassing that of Cassini,
two worlds in a single relatively low cost mission”

Credit: Guiseppe Mitri and the Explorer of Enceladus and Titan team

For scientists interested in habitability and
life, Titan and Enceladus are obvious targets for exploration.The Cassini spacecraft, which is nearing the
end of its thirteen year exploration of the Saturn system, discovered that both
worlds have salty water oceans beneath icy crusts.Titan also has a rich stew of organic molecules
in its atmosphere that are deposited on its surface and into its methane-ethane
surface seas.Enceladus conveniently has
plumes jetting samples of its ocean into space. Within the plumes, Cassini’s
instruments have found organic molecules and trace minerals suggestive of
hydrothermal water-rock interactions that could provide a habitat for microbes.

The three proposed missions to Enceladus and
Titan would employ relatively simple spacecraft with just two to three
instruments.They stand in contrast to
NASA’s planned mission to explore the habitability of Europa, another ocean
world.The Europa mission will bristle
with nine instruments, several of which will produce floods of data that
require a high-powered communications system to return the data to Earth.

A key difference between the Europa and the
Saturn moons missions is the prior history of exploration.Europa was explored by the Galileo spacecraft
built with 1970s technology that had a crippled communications system.As a result, NASA’s new Europa mission must conduct
its own comprehensive global study of this world.At Saturn, this initial investigation of
Enceladus and Titan has been conducted by the highly capable Cassini spacecraft
built with 1990s technology.As a
result, the next mission to these moons can focus on a few specific questions answerable
with two to three instruments.

(For those of you who recall that the Galileo
spacecraft explored Jupiter’s system in the 1990s, the completed spacecraft sat
on the ground for almost a decade due to launch delays.)

Two of the E2T spacecraft’s instrument
would focus on in situ composition
measurements.At Enceladus, the spacecraft
can fly through the plumes and directly taste the ocean’s contents.At Titan, complex organic molecules are
carried to the outermost fringes of the atmosphere where the spacecraft can
sample the atmosphere’s chemistry.

The E2T spacecraft’s two mass
spectrometers would sample material during each passage and determine their
composition by “weighing” their constituent molecules.The distribution of the weights of the
different molecules can be interpreted to determine the composition of the
original material.The spacecraft’s ion
and neutral gas mass spectrometer would determine the composition of gasses
while the Enceladus Icy Jet Analyzer would determine the composition of ice,
salt, and dust particles in Enceladus’ plumes.The two mass spectrometers will measure the nature of the organic
chemicals, the pattern of carbon isotopes, the relative abundances of noble
gases, and search for amino acids and abnormal isotope rations in organic
molecules that suggest a biological origin.

The Cassini spacecraft carried versions of
both these instruments, but the E2T instruments would have a forty
to fifty times improvement in resolution (the ability to distinguish similar
molecules) and sensitivity (the ability to measure minute amounts of a
substance) over their predecessors.(NASA’s Europa mission would carry similar instruments to E2T’s
as will Europe’s JUICE mission to Jupiter and its moon Ganymede).

The E2T team has several key
questions that the two mass spectrometers would address.For Enceladus, are the materials in the plume
most likely from its formation or from current geological or biological
processes?What does the composition reveal about the
nature of the liquid reservoir (currently believed to be a global ocean beneath
an icy cap) and its potential as a habitat for life?For Titan, what are the sources of its
volatiles and how they have been subsequently processed?Does the atmospheric composition suggest that
the current atmosphere is refreshed by material reaching the surface from the
deep water ocean?

The E2T’s spacecraft’s third
instrument would image Titan’s surface and the sources of Enceladus’ plumes
(the so called ‘tiger stripes’) in the near- and short-wave infrared.(The instrument’s name spells out TIGER for
the Titan Imaging and Geology, Enceladus Reconnaissance camera.)Titan
is perpetually shrouded in atmospheric haze, hiding the surface from cameras
that image in most wavelengths.However,
spectral windows at 1.3, 2, and 5 microns allow a camera to image the surface
at several times finer resolution than a similar instrument on the Cassini
spacecraft (and also at better resolution than Cassini’s radar images).The images returned by the camera will
address several key questions such as: To what degree are sediments produced
and transported by fluvial and aeolian processes?How have the rivers and seas of liquid methane
and ethane modified the surface?How
does the composition of the surface, revealed by the three ‘colors’ of the
spectral bands, vary?

If the E2T mission is selected as a
finalist in ESA’s medium class competition, the team will investigate whether a
radio science experiment to study the two moons’ gravitational field could be
added.If it is, this experiment will
investigate the thickness and mechanical properties of the ice shell at
Enceladus’ southern pole where the plumes originate.The gravity measurements would also be used
to investigate Titan’s ice shell and the properties of its internal ocean.

The E2T team hasn't release a
description of its planned imaging coverage of Titan's surface or flights through the plumes of Enceladus. However, the similar JET proposal would have imaged large portions of Titan's surface at several times the resolution of Cassini's instruments and flown through the plumes at different altitudes. Credit: JPL/Caltech

JET

ELF

E2T

Neutral
and ion mass spectrometer

X

X

X

Particulate
mass spectrometer

X

X

Infrared
camera

X

X

Comparison
of instruments proposed for the three proposed Saturn orbiters that would
perform multiple flybys of Enceladus and Titan.The E2T team is proposing a more ambitious mission than did
the preceding teams.

If Titan and Enceladus are obvious targets for
further exploration, what is surprising about E2T and its
predecessor proposals is that they were put forward for the lowest-cost class
of missions flown by the European Space Agency (ESA) and NASA to explore the
planets.

Both ESA and NASA have mission categories for similarly
priced low cost planetary exploration.For the Europeans, this is the Medium class mission program where
missions can be proposed for planetary science, astrophysics, or
heliophysics.For the Americans, this is
the Discovery program that is dedicated to the exploration of the solar system.These programs cap the cost for costs
directly managed by the mission’s principle investigator at €550 million for
ESA and $450M for NASA.(A Euro spent in
Europe and a dollar spent in the United States have similar purchasing
power.)For the ESA missions, the cap must
cover the spacecraft, mission operations, and launch but doesn’t include the
costs of instruments, science teams, or data analysis, which are paid for
separately by individual nations.For
NASA missions, the cap must cover the spacecraft, instruments and the science
team (except those contributed by foreign nations) but doesn’t include launch
or mission operations, which are paid for separately by NASA.(The difference in the way instrument costs
are accounted for by ESA and NASA explains why European mission proposals
typically include more instruments than similar American proposals.For European missions, instruments are off
the PI’s budget.US proposers like to
include foreign instruments paid for by their governments because they again
are off the PI budget.)

Once all costs are added up, these missions typically
cost somewhere in the neighborhood of 600 to 700 million Euros or dollars. However,
this is still only about 60% of the cost of the cheapest outer planets mission
to date, the Juno Jupiter orbiter, at $1.1 billion.(The Juno spacecraft has to survive Jupiter’s
intense radiation levels, which are absent at Saturn, accounting for a portion
of its greater costs.)

Developing a winning proposal for a mission to
Saturn for less than €700 million is challenging.Per the abstract for an upcoming conference,
the JET and ELF missions “were rejected [by NASA] for too-high cost risk.”

The E2T proposal, however,
incorporates the lessons learned from those earlier proposals, and scientists from
the earlier efforts are members of the E2T team.We aren’t privy to all the details of how the
E2T team proposes to fit within the budget cap.This information is a key part of the proposal’s
competitive edge.(The presentation on
the mission supplied to me by Dr. Mitri, for example, has a number of figures
removed because they would reveal too much about key details of the proposal.)However, the team has shared enough
information about the mission to reveal some of the key strategies.

As described above, the E2T would
have limited, focused science goals allowing for a simple, low-cost
spacecraft.Once at Saturn, the
spacecraft would orbit Saturn with six flybys of Enceladus and seventeen flybys
of Titan over 3.5 years.In the weeks between
encounters, the spacecraft would leisurely return each flyby’s data to
Earth.This enables a lower peak data
return rate, which ripples through the spacecraft’s design, lowering mission
costs.Having just three instruments and
substantial time between encounters also reduces the number of mission
controllers needed, again reducing costs.

The E2T team hasn't release a description of its planned orbits around Saturn to explore Enceladus and Titan. However, the similar JET proposal would have used orbits to encounter Titan in multiple locations to allow imaging of different hemispheres. A series of orbits would have provided flights through Enceladus' plumes. Credit: JPL/Caltech

The E2T team proposes to use a
shared launch to a geostationary transfer orbit with another spacecraft such as
a communications satellite.Splitting
the launch cost would save the E2T up to 60 million Euros, or enough
to pay for much of the cost of mission operations on the way to Saturn.This strategy is enabled by using solar
electric propulsion, which allows the spacecraft to propel itself out of Earth
orbit and reach Saturn in just six years.(The JET mission would have required 7 to 8.5 years and ELF ~10 years to
reach the ringed world.)Once at Saturn,
the spacecraft would use chemical propulsion to enter orbit and set up its
encounters.

Further cost savings would come from using
solar power to generate electrical power instead of the more costly
plutonium-powered radioisotope generators.(Although part of the reason for this choice is that ESA lacks the
technology for radioisotope generators and US law prohibits NASA from supplying
one for a foreign spacecraft.)By using
massive solar arrays of 160 square meters (compared to around 70 square meters
for the Juno spacecraft), the spacecraft can create a comfortable 620 Watts of
power at Saturn.While a portion of this
power will need to go to heaters to keep the spacecraft and its instruments
warm (around half of Juno’s power is reserved for this), the E2T
mission would have ample power margins.

The E2T spacecraft would use massive solar arrays to gather enough sunlight at Saturn to produce electrical power. Credit: Guiseppe Mitri and the Explorer of Enceladus and Titan team

It is these large solar arrays that enable the
spacecraft to return the data from mapping Titan’s surface.The ELF mission, by comparison, would have
had smaller solar arrays and would have lacked the power to return imaging data.(The JET mission would have used radioisotope
generators, which would have supplied enough power, and spare heat, to enable the
mapping of Titan.)

The E2T proposal also has another
advantage over its predecessors.If the
mission is selected by ESA, the team would have almost five years to refine the
design before the actual implementation begins.This is more time to fine tune cost savings and to take advantage of
advances in spacecraft technology that have the potential to reduce costs.NASA’s Discovery missions, by contrast, have
to be ready to begin implementation immediately after their selection.As a result, the proposing teams must have
resolved all key design issues at the time of the proposal.

The most encouraging sign for me, though, about
the E2T concept is that the community of scientists interested in
exploring Titan and Enceladus continue to put forth similar if more refined proposals.Preparing for these mission competitions is
time consuming and expensive.If the
budget math wasn’t close, I don’t believe they would keep coming back.Another fact to consider is that in NASA’s
Discovery program, teams often propose missions several times, incorporating
the lessons learned each time their proposals are passed over.The E2T team builds on the
experience of two previous proposals.

If the E2T proposal is passed over,
the Discovery program may hold another lesson.The team behind the OSIRIS-REx mission on its way to do a sample return
from the asteroid Bennu twice unsuccessfully proposed their ideas to the
Discovery program.Their efforts finally
succeeded when they proposed to NASA’s New Frontiers program, which at the time
allowed PI costs of around $800M.NASA
has begun the competition for its fourth New Frontiers mission, and proposals
for missions to Titan and Enceladus are requested (along with proposals for
missions to five other solar system destinations).I expect that at least one mission similar to
E2T, perhaps with more instruments, will be proposed.(We may also see teams used the additional
funding available to propose a sample return from Enceladus’ plumes or a Titan
lander.At the time of the JET proposal,
a Titan lake lander also was proposed for the Discovery program but not
selected.)

ESA will announce the list of finalists for
its competition this June, NASA will announce its list in November.If selected, an ESA mission to Titan and
Enceladus would launch around 2030 while a NASA mission would launch around
2025.These two competitions likely represent
the best hope for fans of these two worlds to see a spacecraft arrive at Saturn
by the mid-2030s.

2 comments:

In the next decade or so we will have:BepyColombo on MercuryVenera-D and Indian mission on Venusat least 2 sample returns and a lot more on the MoonExoMars Nasa 2020 rover and a lot more on Marsseveral small body missionsJuice and American Europa missions on JupiterWith this on Saturn, hopefully Nasa flagship mission to ice giants and New Horizons still going strong behind Pluto I would be happy.And with China getting stronger, hopefully stronger Russia and countries like South Korea, UAE, Iran and new ESA members joining the party this could be really interesting next 10-15 years.Venus untill recently seemed to be neglected but withmany flybys by BepiColombo, solar orbiter and solar probe plus, Venera-D getting serious and with what seems to be capable Indian mission things are looking brighter. And China is bound to have a Venus mission sooner rather than later.

About Me

You can contact me at futureplanets1@gmail.com with any questions or comments.
I have followed planetary exploration since I opened my newspaper in 1976 and saw the first photo from the surface of Mars. The challenges of conceiving and designing planetary missions has always fascinated me. I don't have any formal tie to NASA or planetary exploration (although I use data from NASA's Earth science missions in my professional work as an ecologist).
Corrections and additions always welcome.